This is a phase I/II study to determine the safety, pharmacokinetics and antiviral activity of efavirenz in combination with nelfinavir and nucleoside reverse transcriptase inhibitors in HIV-infected children. Children enrolled in Cohort I will receive efavirenz capsules, while those enrolled in Cohort II will receive efavirenz liquid suspension. For both cohorts, pharmacokinetic studies were obtained for efavirenz and nelfinavir at weeks 2, 6 and, if needed, week 10, and doses were adjusted if levels were outside of target ranges. Subjects were monitored for safety parameters, viral load, CD4 and CD8 cell counts, activation markers on CD8 cells, and lymphocyte proliferation to recall and HIV antigens. Cohort I has been enrolled fully. Enrollment of children > 2 years of age into Cohort II has been completed; enrollment of younger children will begin in the near future.